1. Field of the Invention
The present invention is directed to an all-fiber saturable absorber (SA) and various laser designs employing the same. The saturable absorber is constructed using a fiber taper embedded in a nanostructure/polymer composite.
2. Description of the Background Art
Mode-locked lasers producing femtosecond pulses have found numerous applications in material processing, supercontinuum generation, ultraprecise frequency measurement, and many other significant areas of modern technology. Recently, mode-locked fiber lasers have attracted considerable attention due to the possibility of producing subpicosecond pulses in a compact, low-cost package. Many commercial mode-locked fiber lasers use semiconductor saturable absorber mirrors (SESAMs) for initiating and stabilizing the pulses inside the laser cavity. However, a SESAM is a complex and expensive device, which also requires cumbersome alignment, thus reducing the advantages of an all-fiber format.
In the past few years, a new type of fast saturable absorber based on single-walled carbon nanotubes (SWCNTs) has been discovered and extensively investigated. The advantages of the new saturable absorber include ultrafast saturation recovery time (≈800 fs), a wide working wavelength range, a high damage threshold, and low cost. Different methods of incorporating SWCNT-based saturable absorbers inside laser cavities (both fiber and bulk solid-state lasers) have been proposed, and different results for mode-locked lasers have been demonstrated. The majority of the proposed techniques, however, have unavoidable drawbacks, especially when applied to fiber lasers. For example, saturable absorbers based on
SWCNTs coated directly on one of the laser cavity's mirrors require additional alignment and protection. When SWCNTs are incorporated between two fiber ferrules in a standard fiber connector, there is physical contact between the tubes and the polished fiber ends, making the device vulnerable to damage and difficult to control. Saturable absorbers applied to D-shaped fibers have the advantage of compatibility with the all fiber format, but they turn out to be polarization sensitive. In addition, the process of manufacturing such devices appears to be fairly complex.
Meanwhile, all-fiber lasers have recently attracted significant interest. CW and Q-switched fiber lasers are now capable of achieving high average power, comparable to or even better than traditional solid-state lasers such as Nd:YAG. The fiber format offers crucial practical advantages including passive air cooling and freedom from alignment and maintenance. High efficiency combined with the possibility of direct diode pumping make fiber lasers attractive for many applications, especially in material processing.
Recent progress in thulium (Tm) and holmium (Ho) doped fiber lasers has enabled the demonstration of high-power fiber lasers in the mid-infrared region. Average powers up to 100 W and 68% efficiency have been demonstrated in Tm- and Ho-doped fiber lasers. The possibility of achieving quantum efficiency greater than one due to cross-relaxation energy transfer processes makes this class of fiber lasers very attractive. Furthermore, for many applications in nonlinear optics, medicine and sensing, integrated and robust laser sources around 2 micron wavelength are needed. Tm-doped fiber is known to have a broad and smooth fluorescent spectrum, which is suitable for generating ultrashort pulses. However, only a few mode-locked oscillators based on Tm fiber have been reported.
Nonlinear polarization evolution (NPE) has been used to demonstrate a mode-locked thulium fiber laser that generated 500-fs pulses. A semiconductor saturable absorber mirror (SESAM) has been used in a Tm fiber laser to achieve 190-fs pulses. In ultrafast fiber lasers, NPE and SESAMs are widely used to provide amplitude modulation. However, these two techniques have drawbacks. NPE requires additional elements in the cavity, including a polarizer and polarization controllers. Furthermore, fiber lasers mode-locked with NPE will generally not be environmentally-stable. SESAMs have recently become readily-available, but tend to be damaged in fiber lasers, perhaps owing to the large modulation depth that is needed.
The development of new pulse-shaping mechanisms in fiber lasers allow the generation of high-energy femtosecond pulses. In stretched-pulse, self-similar and wave-breaking-free mode-locked lasers some degree of dispersion compensation is needed. When the laser is designed to operate around 1 μm wavelength, dispersion compensation is implemented either with bulk components, or with microstructured fibers or chirped fiber Bragg gratings. The former approach produces the best performance but undermines some of the benefits of fiber. The latter approach improves the integration but typically results in major performance sacrifices.
Recently, a Yb-doped femtosecond fiber laser was demonstrated without anomalous dispersion in the cavity. The pulse-shaping mechanism is based on strong spectral filtering of a highly-chirped pulse in the laser cavity, and the pulses are dissipative solitons. Pulse energy as high as 26 nJ has been demonstrated with this design. However, bulk components were also present in the laser cavity, which again sacrificed the advantages of fiber format.
All-fiber versions of the normal-dispersion lasers should have tremendous potential for applications. Several groups have investigated all-fiber lasers at 1 μm wavelength. Of course, dispersion control is not required for saturable-absorber mode-locking with picosecond pulse durations; under these conditions, dispersion and nonlinear phase modulation do not contribute appreciably to pulse-shaping. A simple dispersion-compensation-free laser which generates low-energy picosecond pulses has been demonstrated in which a SESAM with high modulation depth shapes the pulses. In another all-fiber oscillator, mode-locking through NPE used a Faraday mirror was used in combination with angle-splicing of PM fibers.
Saturable absorbers based on fiber tapers coated with SWCNTs appear to be the best solution for fiber-based femtosecond pulsed laser applications. These devices are polarization insensitive, and their fabrication process is not complicated. Nevertheless, saturable absorbers based on fiber tapers with SWCNTs directly sprayed on the taper's surface exhibit significant losses caused by scattering. Due to this drawback, the saturable absorber would work only if the SWCNTs were deposited on a very short length at a designated area of the taper.